Plastic lenses have recently rapidly been widespread as optical materials for eyeglasses, camera lenses, and the like because the plastic lenses are lightweight and less broken and can be dyed, as compared with inorganic lenses.
The performances required for the plastic lenses include optical performances such as a high refractive index and a high Abbe's number, and physical and chemical properties such as high impact resistance, ease of dyeing, no problem about heat resistance, and a low specific gravity. The performances further include the property that the lens producing method and monomer compound used have safety of the human body and ease of handling.
Of these performances, high heat resistance and a low specific gravity are realized at a high level even by using an existing high-refractive-index plastic lens. An example of resins which are currently used for a variety of plastic lenses is a resin produced by radical polymerization of diethylene glycol bis(allyl carbonate) (referred to as “D. A. C” hereinafter). This resin has various properties such as excellent impact resistance, a light weight, excellent dyeing performance, good processability such as cutting ability, polishing ability, and the like, etc. However, this resin has a refractive index nd of as low as about 1.50, and thus the central thickness and edge thickness of a plastic lens are increased. Therefore, a resin having a higher refractive index is demanded for the plastic lenses.
Known examples of a resin having a higher refractive index than that of the D. A. C resin include sulfur atom-containing resins such as a polythiourethane resin, a sulfur-containing o-(meth)acrylate resin, and thio(meth)acrylate resin. The polythiourethane resin is a well-balanced resin having a high refractive index, excellent impact resistance, excellent dyeing performance, and the like. For other resins having a high refractive index and high Abbe's number, a method using a polyepisulfide compound is proposed (refer to, for example, Publication No. WO89/10575, Japanese Unexamined Patent Application Publication Nos. 9-110979 and 11-322930).
Also, a method of adding a known sulfur-containing compound to a polyepidithio compound or polyepisulfide compound is proposed for achieving a higher refractive index. However, the producing method and identification data for the compound are not described in examples, and a method of adding a known sulfur-containing compound to the polyepisulfide compound decreases the crosslinking property of a resin to decrease heat resistance. Therefore, this method has low practicability (refer to, for example, Japanese Unexamined Patent Application Publication Nos. 2000-281787, 2001-002783, and 2002-040201).
In the above-described method of adding a known sulfur-containing compound to the polyepisulfide compound, an example of compounds to be added is a monofunctional thietane compound. A typical known example of the thietane compound is methacryloyloxyalkylthietane which is used as a photopolymerizable compound for a photosensitive composition. This composition has a low refractive index and no transparency, and thus cannot be used for some optical applications required to have such a high refractive index as in the present invention (refer to, for example, Japanese Unexamined Patent Application Publication Nos. 55-066909 and 59-180544).
In the above-described conditions, the proposed method using the polyepisulfide compound having a high refractive index and high Abbe's number in a well-balanced state has been advanced to practical use. However, the method causes a handling problem due to the low thermal stability of the polyepisulfide compound in some cases. In order to solve the problem, a method for improving the thermal stability of the polyepisulfide compound is required, and various methods have been proposed. However, any one of the proposed methods is not satisfactory (refer to, for example, Japanese Unexamined Patent Application Publication No. 11-256038).
Furthermore, a resin produced by curing the polyepisulfide compound is brittle, and thus cannot be subjected to special processing such as two-point processing or the like, particularly, in the application to eyeglasses. Also, in some cases, there is the problem of cracking a lens in releasing after casting polymerization. Furthermore, in some of the applications to eyeglasses required to have safety, necessary physical properties are not sufficiently satisfied because of the low impact resistance of the resin. Therefore, various methods have been proposed for improving the resin, but any one of the methods is unsatisfactory (refer to, for example, Japanese Unexamined Patent Application Publication Nos. 2001-131257).
In a condition in which a plastic lens is required to have a higher refractive index and higher Abbe's number, it is greatly demanded to propose a novel compound which can be replaced with the polyepisulfide compound, which has a high refractive index and high Abbe's number, and which permits improvements in brittleness and impact resistance.
Therefore, the inventors conducted studies for improving the refractive index of a compound different from the polyepisulfide compound without decreasing the Abbe's number to provide a material complying with the requirements of a higher refractive index and higher Abbe's number.